注意按学校升序（默认）排序
-- 子查询
SELECT a.device_id
      ,a.university
      ,a.gpa
from user_profile a
where gpa in (
              select min(b.gpa)
              from user_profile b 
              where a.university = b.university )
order by a.university

-- 子查询进阶
SELECT a.device_id
      ,a.university
      ,a.gpa
from user_profile a
where (university,gpa) in (
              select b.university  -- 不是纯聚合字段，需要配合group by
                    ,min(b.gpa)
              from user_profile b 
              group by b.university )
order by a.university

-- 使用all子查询
SELECT a.device_id
      ,a.university
      ,a.gpa
from user_profile a
where gpa <= all (
              select b.gpa
              from user_profile b 
              where a.university = b.university )
order by a.university